This invention relates to an electric thermal unit for controllably heating cylinders in extruders and injection presses for plastic materials, rubber drawing machines, and the like apparata, having two coaxial interspaces for circulating ventilating air therethrough.
As is known, some processing operations of plastomeric and elastomeric materials in general require the availability of specific equipment (such as extruders, injection presses, and drawing machines) comprising essentially a screw arranged to turn with a little play within a coaxial cylinder.
Also known is that said cylinder must be provided with a heating means, controlled by some suitable automatic heat regulating systems, effective to promote proper sonftening of the material prior to its being forced through a forming die (or die plate).
Such heating means usually comprises spiral coil resistance heaters which, being laid between suitably shaped ceramic supports, encircle the cylinder.
More precisely, such electric resistance heaters and related insulating supports are held in a ring-like structure which comprises two or more hingedly interconnected parts and is adapted to fit closely against the outer surface of said cylinder.
It should be noted, however, that said ring-like structure enclosing the resistance heater is, in turn, subjected to undesired surface heating and may, accordingly, become a potential risk for the machine operators.
Further, in view of heat tending to flow upwards, the cylinder temperature is at all times slightly above that of the cylinder lower portion.
This circumstance may originate disuniformity in the mixture and result in finished products which fall short of expectations.
It should be further remembered that, sometimes, conventional electric thermal units incorporate ventilation systems effective to promote containment of the cylinder temperature within more suitable ranges for the intended processing operations.
Such prior ventilation systems usually include a jacket surrounding the electric thermal unit, wherethrough air is circulated to just provide local cooling which may, therefore, enhance any temperature differential existing between the cylinder upper and lower portions.